In the design of a variety of optical analytical instruments, a device called an "integrating sphere" is often utilized. This device usually consists of a hollow sphere with the hollow coated with a highly reflecting substance. When handling the visible part of the spectrum, barium titanate or some zinc zirconates are often used, and when handling longer wavelengths, from the infrared to the microwave range, gold and other highly conducting metals can be used. In both cases it is usually preferred that the reflective surface be relatively rough, namely, are composed of small surfaces that are not all continuously change their orientations with the curvature of the substrate (like in a polished surface; this permits true diffusion of incident light on these surfaces and thus remove a high percentage of specularly reflected light that usually decreases the signal-to-noise ratio of such a sphere.
This can be achieved by a number of different pre-treatments of the substrates, all well known in the prior art of integrating spheres.
The efficiency of such integrating spheres is directly related to the reflectance of the coating of the hollow and, when optimization of signal-to-noise ratio is required, or there is to be maximization of recapture of illumination entering the sphere, a perfect reflector would be the most efficient inner coating of such an integrating sphere.
In my above-identified prior co-pending application entitled "Superconducting Mirrors" I have described the principles of making and using mirrors with extremely low losses, at least to wavelengths corresponding to the superconductor's band gap. In the same application I have mentioned that superconductors of the type (SC,I), (namely, the normal state is an semiconductor or a semimetal), could have band gaps large enough that reflection in the visible part of the spectrum may be made possible. This situation is postulated for materials where a virtual T(C) is much larger then the observed T(C).